Which, from the Chaos side of things, basically saved the whole thing, because there were several critical fails on the part of the Ruinous Powers - Angron, Magnus and Night Haunter set off this domino effect of precognition and long-range psionic warnings that meant the Imperium knew trouble was coming. Horus, Sanguinius and El'Jonson were at one point holed up in secret trying to work out whether to run like hell or die fighting. Dorn turning, which *nobody* expected, was what carried the Heresy.